How does monitor size work?

[Cannon]

Does this site go by the actually viewable size or the rounded specifications? My monitor is labeled as a 25 inch but its actual value in diagonal inches is 24.5

[DPI Wizard]

If you enter the same monitor size for input and output game it doesn't matter, it is not used for the sensitivity calculation.

If you enter a different size the calculator will do a conversion, and the correct size to use is the actual diagonal size for most accurate results.